Mosaic photoelectric target

ABSTRACT

An electro-optical imaging device has a target with an imagereceiving surface (e.g., electrons or photon sensitive) primarily designed to be disposed within an image which is substantially spatially fixed with respect to the surface and has a substantially time invariant outline.

Umted States Patent 11 1 1111 3,894,259

Webb July 8, 1975 [54] MOSAIC PHOTOELECTRIC TARGET 3,405,309 10/1968 Goetze et a1 313/67 X 3,423,623 1/1969 Wendland 313/66 x 175] Inventor: Robert Webb Lexmgmn Mass- 3,564,321 2 1971 Saldi 313/67 x 73 Assignee; Block Engineering, Inc" Cambridge 3,585,439 6/1971 Schneeberger 315/ 11 Mass 3,707,657 12/1972 Veith 317/235 AK [22] Filed: 1973 Primary ExaminerA1fred L. Brody [21] Appl. No.: 321,748 Attorney, Agent, or FirmSchiller and Pandiscio 152] US. Cl. 313/368; 313/374; 315/10; 57 ABSTRACT 357 56 51 Int. Cl. H01 j 31/26 electm'cptical imaging device has a target with [58] Field of Search H 315/10 11 12; 313/65 A image-receiving surface (e.g., electrons or photon sen- 313/ 7 3 sitive) primarily designed t0 be disposed within an 357/56 image which is substantially spatially fixed with respect to the surface and has a substantially time invari- [56] References Cited ant Outlme' UNITED STATES PATENTS 9 Claims, 4 Drawing Figures 3,401,293 9/1968 Morris 315/12 6 Lu SPECTRAL SOURCE 3 I w ('62 SCAN CONTROL COMPUTER POWER SOURCE PKTENUZH UL 8 1975 SPECTRAL SOURCE SCAN CONTROL COMPUTER 4 A E m0 Dn U O 8 6 5 8 W D I A MOSAIC PI-IOTOELECTRIC TARGET This invention relates to electron discharge devices and more particularly to electron or photon sensitive target electrodes for electron tube such as secondary electron vidicons, silicon vidicons, image orthicons or the like.

The present invention is intended to provide an improved target electrode for electro-optical imaging tubes, particularly tubes of the type employing an electron gun for directing a beam of electrons against a target enclosed in an evacuated envelope. The electrons from the beam establish a charge on the target and when the latter is exposed to an image formed by photons or electrons, depending on the type of target, electron flow occurs through the various areas of the charged target in accordance with the image intensity at those areas. The resulting pattern of charge on the target can be removed from the latter as an electrical signal.

For example, in secondary electron conduction l SEC vidicons the target comprises a photoconductor such as fibrous potassium chloride. An image is formed by photons on a photocathode which emits photoelectrons. These latter in turn form an image on the target, and this last image is amplified by a secondary electron avalanche process. These secondary electrons drain from the target through a conductive electrode plate, leaving a positive charge at each point on the target proportional to the radiant energy flux on that during the exposure period. The positive charge on the target lS trapped in the potassium chloride crystal lattice and it remains stable, for up to several hours, enabling delayed readout. Thus, the SEC vidicon can be used to integrate low level light signals, because each resolution element of the target acts as a detector, amplifier and integrator.

lt has been proposed to employ electro-optical imaging devices, particularly SEC vidicons, as detectors wherein the charge image on the target is read out on a selective point by point basis rather than on the basis of a single raster scan. Thus, assuming the target to be overlaid with a Cartesian grid, a scanning beam can be deflected to successive coordinate (x,y) positions on the target, and the beam being blanked until the desired values of the deflection signals are applied. In one version of the proposed system, the deflection signals are voltages corresponding to the values of digital signals which can be stored and manipulated by digital computer. A special application of this latter system involves dividing an optical spectrum into several segments and projecting these segments in stacked array onto an SEC vidicon target. The wave-numbers or wave-lengths of any selected value can be read from the target by the technique above described. In other tral lines in the presence of strong ones can be improved, as indicated hereinafter.

It will be appreciated that the resolution one obtains using the above-described technique has been limited by several factors: the dark current, the electrical isolation between resolution elements of the target, the integration time and the like.

It will also be appreciated that for other types of electrooptical imaging devices such as orthicons, the image formed by photons is read off directly by a scanning electron beam. Hence the electron or photon sensitive electrodes are herein generically referred to as image translating target electrodes.

The principal object of the present invention is to provide an image translating target electrode of a type which, in use, tends to minimize the above-noted limitations on resolution. Another object of the present invention is to provide a target electrode of the type described wherein the target is a mosaic or array of image translating elements positioned only where the expected image pattern is to be formed or focused.

Other objects of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements, and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims. For a fuller understanding of the nature and objects of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic spectrometer system employing the principles of the present invention, shown partly in block and partly in fragmentary section;

FIG. 2 is a plan view showing an exemplary spectral image formed on the target of the electron discharge imaging device of FIG. 1;

FIG. 3 is a fragment, shown partly in cross-section perspective, of the target of FIG. 2; and

FIG. 4 is a fragment, shown partly in cross-section of another target embodying the principles of the present invention.

The invention finds its more important application in systems wherein an image of relatively time-invariant outline is formed, i.e., the intensity (above some predetermined minimal level) of various points on an image may be variable but the dimension and shape of the image, in its focal plane, or on some target are substantially time invariant as well as spatially fixed within its focal plane. A particular example of such image is that provided by an electromagnetic spectrum wherein (assuming that the device or system providing the spectrum does not move, i.e., the image remains spatially fixed) the location of spectral wavelengths on a given plane are time invariant although the intensity of spectral lines may vary widely from time to time. Thus, the present invention can be advantageously described in connection with a spectrometric system wherein a spectral image is projected or focused upon the target of an electro-optical imaging tube.

Referring now to FIG. 1, there is shown such a spectrometer embodying the present invention and including lens 20 for focusing radiation from spectral source 22 through slit 24 in plate 26. A shutter mechanism, shown generally at 28 is provided for controlling transmission of radiation from slit 24 onto a first mirror 32,

preferably having a spherical reflecting surface, disposed for collimating the light emitted from slit 24. The collimated light from mirror 32 is directed onto a first optical dispersion element, such as prism 30, for dispersing radiation in a spectrum spread along a first axis. The spectrum provided by prism 30 is directed onto a second optical dispersion element such as echelle 34. The latter is preferably blazed to produce a medium number of orders and to provide a resolving power between the echelon and echelette gratings. Echelle 34 is disposed so that it acts to disperse radiation from mirror 32 into a series of orders spread along a second axis perpendicular to the first or dispersion axis of prism 30. Second curved mirror 36 is provided for decollimating and focusing the array of orders from echelle 34 to a focal plane.

The optical system thus far described in connection with FIG. I will be recognized by those skilled in the art as an exemplary crossed-dispersion system of the type which provides a series of spectral image orders each of which is characterized in that it follows approximately the rule that n A k where A is the center wavelength of each order, n is the number of the order, and k is a constant. g

Thus, for example, as shown in FIG. 2, a spectrum may be provided as l2 inches long by prism 30. The spectrum can then be arranged by echelle 34 into an array for example of three overlapping orders. These orders will be approximately 4 inch spectral sections 38 arranged one above the other. Each of the orders will be a different order of the original spectrum provided by prism 30. The array can be arranged readily to be approximately -square" if desired.

At the focal plane to which mirror 36 focuses, there is disposed a faceplate or transparent window 42 of the electro-optical imaging tube 40 so that the spectral array is compactly arranged on the faceplate 42. For the sake of brevity, the showing of the tube and its controls has been simiplified. Tuve 40 is shown as an electron discharge device. preferably of the SEC type of vidicon. comprising a vacuum-tight envelope. A photocathode 44 is supported directly behind faceplate 42. The usual electron gun, focusing electrodes and coils, deflection coils, ion trap screen and the like are included but for clarity in the drawing, not shown. An SEC target 48 is disposed between the gun (not shown) and photocathode 44.

Target 48 includes signal plate 50 which is electrically connected through load resistor 52 to power supply 54 and is connected through capacitor 56 to analog-to-digital converter 58. A computer shown at 60, is connected to tube 40 for turning the tube on and off and for housekeeping i.e.. ascertaining what elements of the camera are in working order, what elements are energized and the like. Computer 60 also is connected through scan control 62 for directing the electron beam of tube 40 to selected points on target 48, for example, in accordance with x and y coordinates or address signals programmed or stored in the computer and converted to correspondingly valued beam control voltages.

The invention is primarily intended, as hereinbefore noted, for use with respect to an optical system which provides an image having an outline which is substantially invariant in time as well as spatially fixed. Hence, as shown in FIGS. 2 and 3, target 48 comprises the usual electrically conductive support or ring 64 surrounding signal plate 50. The latter is here shown formed of two layers, an Al O layer 66 and an aluminum layer 68. Layer 66 is disposed to face photocathode 44. Disposed on layer 68 is a relatively thick layer 70 of low-density KCI. Layers 66 and 68 are, as usual in SEC vidicons, substantially continuous. However, as shown particularly in FIG. 3, layer 70 is formed as a mosaic of small elements 72 assembled to cover only those portions of the target outlined by or corresponding to the image provided by the optical system of the device. In the embodiment shown the optical system includes not only the optical dispersion elements, mirrors and the like but also the photocathode and associated focusing coil and electrodes so that the optical" image actually falling on target 48 is ultimately formed by photoelectron emission from the photocathode. However, as shown particularly in FIG. 3, layer 70 is formed as a mosaic of small elements 77 assembled to cover only those portions of the target outlined by the optical system. Here, of course, the optical system includes the photocathode and associated focusing coil and electrodes so that the optical image will be formed by electron emission from the photocathode.

The invention has been described in conjunction with a vidicon using a KCI electron-sensitive target. The in vention can also be embodied with other types of electrooptical imaging tubes such as image orthicons and other vidicons. For example, one can modify a vidicon having an electron-sensitive target. An enlarged version of such device is shown in FIG. 4 comprising an array or mosaic of silicon mesa diodes 80 formed by, for example, photoetching a semiconductor wafer to limit the array to the area within the outline of the expected optical image.

In operation of the device of FIG. 1 an appropriate array of spectral orders are provided as an image falling on photocathode 44 and will cause emission of corresponding photoelectrons from the photocathode into the tube. These photoelectrons form a similar image falling on target 48. The photoelectrons penetrate both M 0 layer 66 and aluminum layer 68 (which are thus necessarily quite thin) and dissipate most of their energy in the thick photoconductive layer KCl 72 or the like, generating in the latter a number of low energy secondary electrons. The latter are collected on plate 50 when a scanning beam strikes the KCI layer, the potential at the point of impact is brought to ground or the cathode potential of the gun. The change in potential serves to discharge the capacitor and produce a video signal voltage across load resistor 52. The spectral image focussed onto the photocathode is very nearly linear in wavelength and it can be expected that a given spectral line will almost exactly be at the same point on the target every time. Hence, to proceed from one point to another requires reasonably simple computation. If, after being examined by the computer as previously delineated (if only for calibration purposes), the array or spectral pattern has shifted on the photocathode, the original relationship can be restored by a simple linear transformation of the origin of the coordinate system or by a rotation about the origin or both. It will be apparent that the spectrometer shown permits one readily to measure the spectral intensity of a selected number of spectral lines. This, of course, yields tremendous time savings in avoiding examination of all of the remainder of the spectrum which may be of little or no interest.

As a result. the spectrometer which has been shown to permit one to readily measure the spectral intensity of a selected number of spectral lines, is particularly useful in improving the photometric accuracy of weak spectral lines in the presence of strong ones. This result is particularly useful, for example, in the toxicological assay of lead in physiological fluids. ln such a situation, A SEC vidicon of the type described could be designed to include the target elements in which the spectral lines of suspected trace elements. i.e., lead, mercury, cadmium, etc., would fall; and, omit those target elements, where the bright spectral lines, e.g., sodium, would fall. Since the secondary electron beam in a SEC vidicon has limited resolution compared to the optical and primary electron systems, the selection of the position of target elements to coincide with the weaker spectral lines will enable a more accurate photometric reading since the limits of intensity variation of the image spectral lines would be lessened.

It is apparent that an imaging tube having a target shaped to match the image outline will lack imaging versatility. However, the latter is essentially sacrificed for photometric gain. A uniform target contributes dark current from its entire area whilst the target of the invention provides dark current only from the actual image area. The dark current is thus reduced by the ratio of active to total area. For crossed-dispersion spectral analysis of the type shown in FIG. 1, the reduction can be at least lzl00 or even more, a substantial gain in signal-to-noise ratio without any reduction in signal strength. Further, where the image is formed of a number of separated areas, the overall resolution is improved inasmuch as no background noise can be read in to the inactive areas or channels between active areas. This is particularly true where the active area is formed of silicon mesa diodes, for the added etching enhances diode isolation.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense.

What is claimed is:

1. In an electro-optical image device having a target with an image-receiving surface, and an optical system for imaging onto said surface an image which has a substantially time invariant outline and is substantially spatially fixed with respect to said surface so that said outline will cover only a portion of said surface, the improvement characterized in that substantially only the portion of said surface which receives said image is provided with image-translating material.

2. An electro-optical imaging device as defined by claim 1 wherein said each such image translating material is electron sensitive.

3. An electro-optical imaging device as defined by claim 2 wherein said device is a SEC vidicon tube.

4. An electro-optical imaging device as defined in claim 1 wherein said each such image translating material is photon sensitive.

5. An electro-optical imaging device as defined in claim 4 wherein said device is an image orthicon tube.

6. An electro-optical imaging device as defined in claim 4 wherein said device is a silicon vidicon.

7. An electro-optical imaging device as defined by claim 1 wherein said surface is substantially continuous throughout the area provided with said image translating material.

8. An electro-optical imaging device as defined by claim 1 wherein said image translating material comprises an array of discrete elements.

9. An electro-optical imaging device as defined by claim 8 wherein said discrete elements are silicon mesa diodes. 

1. In an electro-optical image device having a target with an image-receiving surface, and an optical system for imaging onto said surface an image which has a substantially time invariant outline and is substantially spatially fixed with respect to said surface so that said outline will cover only a portion of said surface, the improvement characterized in that substantially only the portion of said surface which receives said image is provided with image-translating material.
 2. An electro-optical imaging device as defined by claim 1 wherein said each such image translating material is electron sensitive.
 3. An electro-optical imaging device as defined by claim 2 wherein said device is a SEC vidicon tube.
 4. An electro-optical imaging device as defined in claim 1 wherein said each such image translating material is photon sensitive.
 5. An electro-optical imaging device as defined in claim 4 wherein said device is an image orthicon tube.
 6. An electro-optical imaging device as defined in claim 4 wherein said device is a silicon vidicon.
 7. An electro-optical imaging device as defined by claim 1 wherein said surface is substantially continuous throughout the area provided with said image translating material.
 8. An electro-optical imaging device as defined by claim 1 wherein said image translating material comprises an array of discrete elements.
 9. An electro-optical imaging device as defined by claim 8 wherein said discrete elements are silicon mesa diodes. 